Substrate provided with an alignment mark in a substantially transmissive process layer, mask for exposing said mark, device manufacturing method, and device manufactured thereby

ABSTRACT

A substrate provided with an alignment mark in a substantially transmissive process layer overlying the substrate, said mark comprising high reflectance areas for reflecting radiation of an alignment beam of radiation, and low reflectance areas for reflecting less radiation of the alignment beam, wherein the high reflectance areas comprise at least one substantially linear sub-grating. In one example, a substantially linear sub-grating comprises a plurality of spaced square regions.

This application is a continuation application and claims priority toU.S. patent application Ser. No. 10/960,695 filed Oct. 8, 2004 now U.S.Pat. No. 7,056,805, which is a divisional application of U.S. patentapplication Ser. No. 10/151,317, filed May 21, 2002 and now patented asU.S. Pat. No. 6,858,948, which claims priority to EP 01201956.8, filedMay 23, 2001, each of which are herein incorporated by reference.

FIELD

The present invention relates to alignment marks.

BACKGROUND

In general, a substrate may be processed in a lithographic projectionapparatus comprising: a radiation system to supply a projection beam ofradiation; a support structure to support patterning structure, thepatterning structure serves to pattern the projection beam according toa desired pattern; a substrate table to hold a substrate; and aprojection system to project the patterned beam onto a target portion ofthe substrate.

The term “patterning structure” as here employed should be broadlyinterpreted as referring to structure or means that can be used to endowan incoming radiation beam with a patterned cross-section, correspondingto a pattern that is to be created in a target portion of the substrate;the term “light valve” can also be used in this context. Generally, thesaid pattern will correspond to a particular functional layer in adevice being created in the target portion, such as an integratedcircuit or other device (see below). Examples of such patterningstructure include:

-   -   A mask. The concept of a mask is well known in lithography, and        it includes mask types such as binary, alternating phase-shift,        and attenuated phase-shift, as well as various hybrid mask        types. Placement of such a mask in the radiation beam causes        selective transmission (in the case of a transmissive mask) or        reflection (in the case of a reflective mask) of the radiation        impinging on the mask, according to the pattern on the mask. In        the case of a mask, the support structure will generally be a        mask table, which ensures that the mask can be held at a desired        position in the incoming radiation beam, and that it can be        moved relative to the beam if so desired.    -   A programmable mirror array. One example of such a device is a        matrix-addressable surface having a viscoelastic control layer        and a reflective surface. The basic principle behind such an        apparatus is that (for example) addressed areas of the        reflective surface reflect incident light as diffracted light,        whereas unaddressed areas reflect incident light as undiffracted        light. Using an appropriate filter, the said undiffracted light        can be filtered out of the reflected beam, leaving only the        diffracted light behind; in this manner, the beam becomes        patterned according to the addressing pattern of the        matrix-addressable surface. An alternative embodiment of a        programmable mirror array employs a matrix arrangement of tiny        mirrors, each of which can be individually tilted about an axis        by applying a suitable localized electric field, or by employing        piezoelectric actuation means. Once again, the mirrors are        matrix-addressable, such that addressed mirrors will reflect an        incoming radiation beam in a different direction to unaddressed        mirrors; in this manner, the reflected beam is patterned        according to the addressing pattern of the matrix-addressable        mirrors. The required matrix addressing can be performed using        suitable electronic means. In both of the situations described        hereabove, the patterning structure can comprise one or more        programmable mirror arrays. More information on mirror arrays as        here referred to can be gleaned, for example, from U.S. Pat. No.        5,296,891 and U.S. Pat. No. 5,523,193, and PCT patent        applications WO 98/38597 and WO 98/33096, which are incorporated        herein by reference. In the case of a programmable mirror array,        the said support structure may be embodied as a frame or table,        for example, which may be fixed or movable as required.    -   A programmable LCD array. An example of such a construction is        given in U.S. Pat. No. 5,229,872, which is incorporated herein        by reference. As above, the support structure in this case may        be embodied as a frame or table, for example, which may be fixed        or movable as required.        For purposes of simplicity, the rest of this text may, at        certain locations, specifically direct itself to examples        involving a mask and mask table; however, the general principles        discussed in such instances should be seen in the broader        context of the patterning structure as hereabove set forth.

Lithographic projection apparatus can be used, for example, in themanufacture of integrated circuits (ICs). In such a case, the patterningstructure may generate a circuit pattern corresponding to an individuallayer of the IC, and this pattern can be imaged onto a target portion(e.g. comprising one or more dies) on a substrate (silicon wafer) thathas been coated with a layer of radiation-sensitive material (resist).In general, a single wafer will contain a whole network of adjacenttarget portions that are successively irradiated via the projectionsystem, one at a time. In current apparatus, employing patterning by amask on a mask table, a distinction can be made between two differenttypes of machine. In one type of lithographic projection apparatus, eachtarget portion is irradiated by exposing the entire mask pattern ontothe target portion at one time; such an apparatus is commonly referredto as a wafer stepper. In an alternative apparatus—commonly referred toas a step-and-scan apparatus—each target portion is irradiated byprogressively scanning the mask pattern under the projection beam in agiven reference direction (the “scanning” direction) while synchronouslyscanning the substrate table parallel or anti-parallel to thisdirection; since, in general, the projection system will have amagnification factor M (generally <1), the speed V at which thesubstrate table is scanned will be a factor M times that at which themask table is scanned. More information with regard to lithographicdevices as here described can be gleaned, for example, from U.S. Pat.No. 6,046,792, incorporated herein by reference.

In a manufacturing process using a lithographic projection apparatus, apattern (e.g. in a mask) is imaged onto a substrate that is at leastpartially covered by a layer of radiation-sensitive material (resist).Prior to this imaging step, the substrate may undergo variousprocedures, such as priming, resist coating and a soft bake. Afterexposure, the substrate may be subjected to other procedures, such as apost-exposure bake (PEB), development, a hard bake andmeasurement/inspection of the imaged features. This array of proceduresis used as a basis to pattern an individual layer of a device, e.g. anIC. Such a patterned layer may then undergo various processes such asetching, ion-implantation (doping), metallization, oxidation,chemo-mechanical polishing, etc., all intended to finish off anindividual layer. If several layers are required, then the wholeprocedure, or a variant thereof, will have to be repeated for each newlayer. Eventually, an array of devices will be present on the substrate(wafer). These devices are then separated from one another by atechnique such as dicing or sawing, whence the individual devices can bemounted on a carrier, connected to pins, etc. Further informationregarding such processes can be obtained, for example, from the book“Microchip Fabrication: A Practical Guide to Semiconductor Processing”,Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN0-07-067250-4, incorporated herein by reference.

For the sake of simplicity, the projection system may hereinafter bereferred to as the “lens”; however, this term should be broadlyinterpreted as encompassing various types of projection system,including refractive optics, reflective optics, and catadioptricsystems, for example. The radiation system may also include componentsoperating according to any of these design types for directing, shapingor controlling the projection beam of radiation, and such components mayalso be referred to below, collectively or singularly, as a “lens”.Further, the lithographic apparatus may be of a type having two or moresubstrate tables (and/or two or more mask tables). In such “multiplestage” devices the additional tables may be used in parallel, orpreparatory steps may be carried out on one or more tables while one ormore other tables are being used for exposures. Dual stage lithographicapparatus are described, for example, in U.S. Pat. No. 5,969,441 and PCTPatent Application WO 98/40791, incorporated herein by reference.

This invention relates to an alignment mark for use in an alignmentsystem of a lithographic projection apparatus for aligning the patternin the patterning structure to the substrate. This alignment system, forexample, the indirect off-axis alignment system described in PCT PatentApplication WO 98/39689 incorporated herein by reference, employs analignment beam of radiation that is radiated by a separate alignmentunit and that is incident on a mark, in the form of a grating on thesubstrate. The grating diffracts the alignment beam into a number ofsub-beams extending at different angles to the normal of the grating.Said distinct sub-beams will be directed with a lens of the alignmentunit to different positions in a plane. In this plane means may beprovided for further separating the different sub-beams. The lens systemwill also be used to finally image the different sub-beams on areference plate to create an image of the mark. In this reference platea reference mark can be provided and a radiation sensitive detector canbe arranged behind the reference mark. The output signal of the detectorwill be dependent on the extent to which the image of the substrate markand the reference mark coincide. In this way the extent of alignment ofthe mark on the substrate with the reference mark in the alignment unitcan be measured and optimized. The detector may comprise separateindividual detectors for measuring the intensity and the alignedposition at different orders. To finish the alignment, the reference inthe alignment unit has to be aligned to a second reference mark, forexample, one provided to the substrate table with the alignment unit.This second reference mark may then be aligned to a mark in the maskusing exposure light. An apparatus and method as described in U.S. Pat.No. 5,144,363, incorporated herein by reference can be used for thatpurpose.

Alternatively, a direct on-axis alignment system can be used thatdirects an alignment beam directly upon a mark provided on the substratevia the projection system. This beam will be diffracted by the mark onthe substrate into different sub-beams and will be reflected into theprojection system. After traversing the projection system the differentsub-beams will be focussed on a reference alignment mark provided to themask. The image of the substrate mark formed by the sub-beams can beimaged upon the reference mark in the mask. In this way the extent ofalignment of the mark on the substrate and the reference mark in themask can be measured and optimized. This can be done by using aradiation sensitive detector constructed and arranged to detect thealignment beam traversing the mark in the mask. For more informationwith respect to the on-axis alignment system here described see, forexample, U.S. Pat. No. 4,778,275 incorporated herein by reference.

To improve the speed of integrated circuits produced with thelithographic projection apparatus, it is proposed to use low resistancematerials, such as copper, as a conductor to decrease time delays in thecircuits. To fabricate integrated circuits with copper, a new substrateprocessing technique known as copper dual damascene is introduced in themanufacturing of integrated circuits. One of the problems that occurswith this new processing technique is that during alignment to marksembedded in a copper dual damascene layer a weak or even no signal ismeasured by the alignment system. It is discovered that the radiation ofthe alignment beam traversing through the mark and the copper dualdamascene layer to the substrate surface may reflect thereupon and maytraverse back to the alignment mark where it may interfere withradiation of the alignment beam directly reflected upon the mark. Theproblem is caused by the layer of copper dual damascene beingsubstantially transmissive for the alignment beam of the alignmentsystem. The interference is largely dependent on the thickness of thecopper dual damascene layer on which the mark is laying. It is howeververy difficult to control the thickness of the layer to such an extentthat the interference can be avoided.

SUMMARY

In an aspect of at least one embodiment of the present invention, thereis provided a mark that can be used in a transmissive process layer andwhich doesn't suffer from a decrease in signal strength caused byinterference of light traversing through the mark and reflecting uponthe substrate surface.

According to at least one embodiment of the invention, a substrate isprovided with an alignment mark in a substantially transmissive processlayer overlying the substrate, said mark comprising:

-   -   relatively high reflectance areas for reflecting radiation of an        alignment beam of radiation; and    -   relatively low reflectance areas for reflecting less radiation        of the alignment beam, wherein the relatively low reflectance        areas comprise scattering structures for scattering and        absorbing radiation of the alignment beam.

According to at least one embodiment of the invention, there is provideda device manufacturing method comprising:

-   -   providing a substrate comprising alignment marks in a        transmissive layer that is at least partially covered by a layer        of radiation sensitive material to a substrate table;    -   aligning the alignment marks comprising relatively high and        relatively low reflectance areas to a reference with an        alignment beam of radiation;    -   providing a projection beam of radiation using a radiation        system;    -   using patterning structure to endow the projection beam with a        pattern in its cross-section; and    -   projecting the patterned beam of radiation onto a target portion        of the layer of radiation sensitive material, wherein the        relatively low reflectance areas comprise scattering structures        for scattering and absorbing the alignment beam.

Although specific reference may be made in this text to the use of theapparatus according to at least one embodiment of the invention in themanufacture of ICs, it should be explicitly understood that such anapparatus has many other possible applications. For example, it may beemployed in the manufacture of integrated optical systems, guidance anddetection patterns for magnetic domain memories, liquid-crystal displaypanels, thin-film magnetic heads, etc. The skilled artisan willappreciate that, in the context of such alternative applications, anyuse of the terms “reticle”, “wafer” or “die” in this text should beconsidered as being replaced by the more general terms “mask”,“substrate” and “target portion”, respectively.

In the present document, the terms “radiation” and “beam” are used toencompass all types of electromagnetic radiation, including ultravioletradiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) andEUV (extreme ultra-violet radiation, e.g. having a wavelength in therange 5-20 nm), as well as particle beams, such as ion beams or electronbeams.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts and inwhich:

FIG. 1 depicts a lithographic projection apparatus in which at least oneembodiment of the invention can be used;

FIG. 2 depicts an alignment mark in which at least one embodiment of theinvention can be used;

FIG. 3 depicts a top view of a part of a sub-grating of the alignmentmark of FIG. 2 according to at least one embodiment of the invention;

FIG. 4 depicts a cross-sectional view of an alignment mark embedded in atransmissive layer on a substrate;

FIG. 5 depicts a top view of a part of a sub-grating of the alignmentmark of FIG. 2 according to at least one embodiment of the invention;and

FIG. 6 shows a detailed top view of a reflective area of a markaccording to at least one embodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic projection apparatus inwhich the substrate provided with the mark according to at least oneembodiment of the invention can be used. The apparatus comprises:

-   -   a radiation system Ex, IL, for supplying a projection beam PB of        radiation (e.g. UV or EUV radiation). In this particular case,        the radiation system also comprises a radiation source LA;    -   a first object table (mask table) MT provided with a mask holder        for holding a mask MA (e.g. a reticle), and connected to first        positioning means for accurately positioning the mask with        respect to item PL;    -   a second object table (substrate table) WT provided with a        substrate holder for holding a substrate W (e.g. a resist-coated        silicon wafer), and connected to second positioning means for        accurately positioning the substrate with respect to item PL;        and    -   a projection system (“lens”) PL (e.g. a refractive or        catadioptric system, a mirror group or an array of field        deflectors) for imaging an irradiated portion of the mask MA        onto a target portion C (e.g. comprising one or more dies) of        the substrate W.        As here depicted, the apparatus is of a transmissive type (i.e.        has a transmissive mask). However, in general, it may also be of        a reflective type, for example (with a reflective mask).        Alternatively, the apparatus may employ another kind of        patterning structure, such as a programmable mirror array of a        type as referred to above.

The source LA (e.g. a HG lamp, an excimer laser, an undulator providedaround a path of an electron beam in a storage ring or synchrotron)produces a beam of radiation. This beam is fed into an illuminationsystem (illuminator) IL, either directly or after having traversedconditioning means, such as a beam expander Ex, for example. Theilluminator IL may comprise adjusting means AM for setting the outerand/or inner radial extent (commonly referred to as σ-outer and σ-inner,respectively) of the intensity distribution in the beam. In addition, itwill generally comprise various other components, such as an integratorIN and a condenser CO. In this way, the beam PB impinging on the mask MAhas a desired uniformity and intensity distribution in itscross-section.

It should be noted with regard to FIG. 1 that the source LA may bewithin the housing of the lithographic projection apparatus (as is oftenthe case when the source LA is a mercury lamp, for example), but that itmay also be remote from the lithographic projection apparatus, theradiation beam which it produces being led into the apparatus (e.g. withthe aid of suitable directing mirrors); this latter scenario is oftenthe case when the source LA is an excimer laser. The current inventionencompasses at least both of these scenarios.

The beam PB subsequently intercepts the mask MA, which is held on a masktable MT. Having traversed the mask MA, the beam PB passes through thelens PL, which focuses the beam PB onto a target portion C of thesubstrate W. With the aid of the second positioning means (andinterferometric measuring means IF), the substrate table WT can be movedaccurately, e.g. so as to position different target portions C in thepath of the beam PB. Similarly, the first positioning means can be usedto accurately position the mask MA with respect to the path of the beamPB, e.g. after mechanical retrieval of the mask MA from a mask library,or during a scan. In general, movement of the object tables MT, WT willbe realized with the aid of a long-stroke module (coarse positioning)and a short-stroke module (fine positioning), which are not explicitlydepicted in FIG. 1. However, in the case of a wafer stepper (as opposedto a step-and-scan apparatus) the mask table MT may just be connected toa short stroke actuator, or may be fixed.

The depicted apparatus can be used in two different modes:

-   1. In step mode, the mask table MT is kept essentially stationary,    and an entire mask image is projected at one time (i.e. a single    “flash”) onto a target portion C. The substrate table WT is then    shifted in the x and/or y directions so that a different target    portion C can be irradiated by the beam PB;-   2. In scan mode, essentially the same scenario applies, except that    a given target portion C is not exposed in a single “flash”.    Instead, the mask table MT is movable in a given direction (the    so-called “scan direction”, e.g. the y direction) with a speed v, so    that the projection beam PB is caused to scan over a mask image;    concurrently, the substrate table WT is simultaneously moved in the    same or opposite direction at a speed V=Mv, in which M is the    magnification of the lens PL (typically, M =¼or ⅕). In this manner,    a relatively large target portion C can be exposed, without having    to compromise on resolution.

For alignment purposes, the mask is provided with marks (M₁ and M₂) inthe mask MA. These marks (M₁ and M₂) may be aligned directly orindirectly through the projection system PL to marks (P₁ and P₂) in thesubstrate W. During this alignment, information will be obtained aboutthe position of the image C projected through the projection system PLupon the substrate W. This is necessary to assure that different layersexposed with different masks are correctly positioned with respect toeach other. It is therefore necessary that before exposure of each layerthe image in the mask MA is aligned to the same substrate marks (P₁ andP₂).

FIG. 2 depicts a substrate alignment mark wherein at least oneembodiment of the invention can be used. Such an alignment mark maycomprise four sub-gratings P_(1, a), P_(1, b), P_(1, c) and P_(1, d), ofwhich the sub-gratings P_(1, b) and P_(1, d) serve for alignment in theX direction and the sub-gratings P_(1, a) and P_(1, c) serve foralignment in the Y direction. The sub-gratings may have a grating periodof, for example, 16 μm. The grating period may be different for eachsub-grating and the dimensions of the sub-grating may be 200*200 μm.Each sub-grating comprises relatively high reflectance areas 3 andrelatively low reflectance areas 1.

It is discovered that by the use of scattering elements in the lowreflectance areas the alignment beam is scattered away so that lessradiation will traverse the transmissive layer to reflect at the surfaceof the substrate and traverse back to the mark to cause interferencewith the alignment beam reflected at the high reflectance areas. Thescattering structures may be provided in the same exposure step in whichthe high reflectance areas are exposed. If the scattering structures ofthe alignment mark are formed as a grating with lines in a firstdirection, the relatively high reflectance areas may be provided as agrating with lines perpendicular to said first direction. The alignmentsystem used for detecting the position of the mark will be sensitive forthe directions of the lines in the grating formed by the relatively highreflectance areas because the light will be mainly diffracted in adirection perpendicular to said lines. The alignment system will not besensitive to radiation diffracted by the scattering structures becausethe direction in which that light is diffracted will be perpendicular tothe direction of the radiation diffracted by the relatively highreflectance areas. The grating period of the grating used as thescattering structure may be smaller than the grating period of thegrating for the relatively high reflectance areas such that theradiation diffracted by the relatively high reflectance areas isdiffracted with a smaller angle to the normal of the mark. The alignmentsystem may be sensitive for radiation diffracted at a certain angle tothe normal of the mark and therefore only be sensitive for radiationdiffracted by the relatively high reflectance areas.

FIG. 3 depicts a top-view on a part of a sub-grating, for exampleP_(1, a), of the alignment mark of FIG. 2. What is shown is four highreflectance areas 3 forming a grating with a grating period in the Ydirection and three low reflectance areas 1. The low reflectance areas 1are provided with a scattering structure having the form of a gratingwith a grating period in the X direction with a small size, for examplea period smaller than 2 μm, for example, 1.14 μm. The function of thescattering structure in the low reflectance areas is to scatter andabsorb the radiation of the alignment beam that is incident upon the lowreflectance areas, so as to prevent radiation traversing through the lowreflectance areas reflecting from the substrate surface and traversingback to the mark to interfere with that part of the alignment beam thatis reflected by the high reflectance areas, causing a disturbance of thealignment signal. The direction of the grating in the low reflectancearea is chosen to be substantially perpendicular to the direction of thegrating of the high reflectance areas so that a part of the alignmentbeam diffracted from the low reflectance areas will be diffracted in adifferent direction than the part of the beam diffracted from the highreflectance area. The part of the alignment beam diffracted from the lowreflectance area will not reach the alignment system, because thealignment system is only sensitive to diffraction in a particulardirection. The diffraction from the low-reflectance areas will thereforenot disturb the alignment system. The period of the grating in thelow-reflectance area is also smaller than the period of the gratingprovided to the high reflectance area. The angle of diffraction with thenormal of the mark will therefore be greater and the chance that theradiation diffracted from the low reflectance area will disturb thealignment system will be further minimized. The high reflectance areasmay have a reflectivity of 50-100% and the low reflectance areas mayhave a reflectivity of 0-10%. A layer of copper dual damascene, forexample, may have a reflectivity of 4%.

FIG. 4 depicts a cross-sectional view along the line 7 in FIG. 3 on analignment mark embedded in a transmissive layer 11 on a substrate W. Thesubstrate W is covered with five process layers (9, 11, 13 15 and 17)that have been exposed with a patterned beam in subsequent exposures onthe substrate W. One of the layers (11) is provided with an alignmentmark having high reflectance 3 and low reflectance areas 1. In case thelayers (9, 11, 13, 15 and 17) are transmissive and no scatteringstructures are provided in the low reflectance areas 1 the radiation ofthe alignment beam from the alignment system may traverse through thelow reflectance areas 1 and may reflect upon the surface of thesubstrate W. Subsequently, the radiation reflected upon the surface mayreach the alignment mark again and interference may occur with that partof the alignment beam that is directly reflected at the mark surface.The scattering features provided in the low reflectance areas accordingto at least one embodiment of the invention will minimize the effects ofreflection at the surface of the substrate.

FIG. 5 depicts a top view on a part of a sub-grating of the alignmentmark of FIG. 2 according to at least one embodiment of the invention.Two high reflectance areas 3 and one low reflectance area 5 are shown.This embodiment(s) shares most items with the embodiment(s) describedearlier, a difference being that the lines of the grating forming thehigh reflectance areas 3 are segmented in two by having a lowreflectance area 21 in the middle of the high reflectance areas 3. Thelow reflectance area 21 is parallel to the lines forming the grating ofthe high reflectance areas 3. The additional low reflectance areaincreases the diffraction in higher orders of the alignment beam by thegrating formed by the high reflectance areas, which is advantageousbecause higher orders give better information of the position of themark. As shown, the high reflectance areas 3 are segmented in two byhaving one low reflectance area 21 in the middle. It is possible thatthe high reflectance areas are segmented in three, four or five parts toimprove the diffraction in higher orders and hence improve the accuracyof alignment.

FIG. 6 shows a detailed top view on a part of a high reflectance area ofa mark according to at least one embodiment of the invention. The highreflectance area is built up out of square surfaces, each square beingrepeated in first and second directions and having sides parallel tosaid first and second directions. As shown here there are two types ofsquare surfaces—large ones 23 and small ones 25—which are repeated toform the high reflectance area 3. The size of the squares is comparablewith the size of the structures that are exposed from the mask onto thesubstrate and there may also be more than two sizes of squares. Acomparable size of the squares in the high reflectance area of the markand the structure to be exposed is advantageous because the diffractionby structures to be exposed is in that case similar to the diffractionby high reflectance areas of the mark. An advantage of this similardiffraction is that beams with a similar diffraction will traverse asimilar optical path through the projection system and therefore willsuffer from the same aberrations in the projection system. Thepositional deviation caused by those aberrations will be similar for thealignment mark and the structures to be exposed leading to abetter-aligned position. The size of the squares may be in the rangefrom 0.05 to 0.5 μm. It must be understood that this at least oneembodiment of the invention also may be used without the use of anyscattering structures in the low-reflectance areas of the alignmentmark, for example directly in the high reflectance areas 3 of thealignment mark of FIG. 2. The at least one embodiment of the inventioncan also be used in a mark suitable for measuring overlay; in that casethe mark may have the form of a large square. The square will in thatcase comprise a large number of smaller squares having a size comparablewith the size of the structures to be exposed.

As an alternative one could measure the position of the differentdiffraction orders in the pupil plane of the projection system PL of aparticular structure to be exposed in the lithographic apparatus. Thepositions in the pupil plane give information of the amount ofdiffraction that occurred with said particular structure. Subsequently,one could measure the position of the different diffraction orders inthe pupil plane of an alignment mark and alter that alignment mark suchthat the position of the different diffraction orders in the pupil planeof the alignment mark and the structure to be exposed become similar.One could also use simulation software for obtaining a mark thatdiffracts to similar position in the pupil plane as a structure to beexposed in the lithographic projection apparatus. Again the highreflectance areas of the mark will be diffracted similar as thestructures to be exposed in the lithographic projection apparatus andwill suffer from the same aberrations in the projection system givingthe same positional deviations and a better aligned position.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The description is not intended to limit theinvention.

1. A mark for use in lithographic overlay measurements, comprising: atleast one relatively high reflectance area to reflect radiation of analignment beam of radiation; and at least one relatively low reflectancearea to reflect less radiation of the alignment beam, wherein the atleast one relatively high reflectance area comprises a plurality ofsubstantially linear sub-gratings, at least one of the substantiallylinear sub-gratings comprising a plurality of spaced square regions. 2.The mark of claim 1, wherein the plurality of substantially linearsub-gratings comprise a plurality of parallel lines.
 3. The mark ofclaim 2, wherein the plurality of substantially linear sub-gratingscomprise four sub-gratings mutually arranged such that lines containedwithin a first sub-grating run substantially perpendicular to linescontained in each sub-grating that is opposed along a common edge to thefirst sub-grating, and wherein the lines within the first sub-gratingrun substantially parallel to lines contained in a sub-gratingdiagonally opposed to the first sub-grating.
 4. The mark of claim 3,wherein the plurality of substantially linear sub-gratings are mutuallyarranged to define a substantially square shape.
 5. The mark of claim 2,wherein each square region has a size in the range of about 0.05 to 0.5micrometers.
 6. The mark of claim 2, wherein the mark is disposed withina layer that is substantially transmissive to an alignment beam used forthe lithographic overlay measurements.
 7. The mark of claim 6, whereinthe layer is a copper dual damascene layer.
 8. The mark of claim 1,wherein the square regions are high reflectance areas.
 9. The mark ofclaim 1, wherein a first substantially linear sub-grating of theplurality of substantially linear sub-gratings is defined by a firstgrating pitch, and a second substantially linear sub-grating of theplurality of substantially linear sub-gratings is defined by a secondgrating pitch different from the first grating pitch.
 10. The mark ofclaim 1, wherein the relatively high reflectance area has a reflectanceof about fifty to one hundred percent, and wherein the relatively lowreflectance area has a reflectance of about zero to ten percent.
 11. Amask comprising a pattern for exposing an alignment mark, the alignmentmark comprising: at least one relatively high reflectance area toreflect radiation of an alignment beam of radiation; and at least onerelatively low reflectance area to reflect less radiation of thealignment beam, wherein the at least one relatively high reflectancearea comprises a plurality of substantially linear sub-gratings, atleast one of the substantially linear sub-gratings comprising aplurality of spaced square regions.
 12. The mask of claim 11, whereinthe square regions are high reflectance areas.
 13. The mask of claim 11,wherein the plurality of substantially linear sub-gratings comprise foursub-gratings mutually arranged such that lines contained within a firstsub-grating run substantially perpendicular to lines contained in eachsub-grating that is opposed along a common edge to the firstsub-grating, and wherein the lines within the first sub-grating runsubstantially parallel to lines contained in a sub-grating diagonallyopposed to the first sub-grating.
 14. The mask of claim 13, wherein theplurality of substantially linear sub-gratings are mutually arranged todefine a substantially square shape.
 15. Simulation software forobtaining an alignment mark, wherein the alignment mark comprises: atleast one relatively high reflectance area to reflect radiation of analignment beam of radiation; and at least one relatively low reflectancearea to reflect less radiation of the alignment beam, wherein the atleast one relatively high reflectance area comprises a plurality ofsubstantially linear sub-gratings, at least one of the substantiallylinear sub-gratings comprising a plurality of spaced square regions. 16.The simulation software of claim 15, wherein the square regions are highreflectance areas.
 17. The simulation software of claim 15, wherein theplurality of substantially linear sub-gratings comprise foursub-gratings mutually arranged such that lines contained within a firstsub-grating run substantially perpendicular to lines contained in eachsub-grating that is opposed along a common edge to the firstsub-grating, and wherein the lines within the first sub-grating runsubstantially parallel to lines contained in a sub-grating diagonallyopposed to the first sub-grating.
 18. The simulation software of claim17, wherein the plurality of substantially linear sub-gratings aremutually arranged to define a substantially square shape.